1. Field of the Invention
This invention is concerned with analysis of electroless plating baths and in particular with determining the stability of such baths against decomposition.
2. Description of the Related Art
Plating baths are widely used by the electronics industry to deposit a variety of metals (copper, nickel, cobalt and gold, for example) on various parts, including circuit boards, semiconductor chips, and device packages. Both electroplating baths and electroless plating baths are employed. For electroplating, the part and a counter electrode are brought into contact with the electroplating bath containing ions of an electrodepositable metal, and the metal is electrodeposited by applying a negative potential to the part relative to the counter electrode. For electroless plating, the bath also contains a reducing agent which, in the presence of a catalyst, chemically reduces the metal ions to form a deposit of the metal. Since the deposited metal itself can serve as the catalyst, the electroless deposition, once initiated, proceeds without the need for an externally applied potential. In immersion plating, substrate oxidation provides the electrons needed to electrolessly deposit a layer of a more noble metal. The immersion plating process is self-limiting, i.e., ceases when the substrate is fully covered with the deposited metal.
Among electroless processes, those for depositing nickel and cobalt are particularly important to the electronics industry, and other industries as well. For example, electroless nickel with an immersion gold surface layer (ENIG) is widely used to provide an oxidation-resistant and solderable/bondable finish on copper circuitry and surface pads on circuit boards, and on aluminum pads on semiconductor chips. As another example, electroless cobalt and nickel processes are used to provide electrically-conductive and conformal barrier and capping layers for copper circuitry on semiconductor chips as part of the well-known “Damascene” process. Electroless cobalt and nickel baths used to deposit Damascene barrier and capping layers typically also contain a refractory metal (e.g., tungsten, molybdenum or rhenium), which co-deposits with the cobalt or nickel and increases the maximum temperature at which effective barrier properties are retained.
For electroless cobalt and nickel baths, hypophosphite (H2PO2−) is typically used as the reducing agent, which introduces phosphorus into the deposit. The codeposited phosphorus reduces the deposit grain size and crystallinity (compared to electrodeposits), which tends to improve the barrier properties and oxidation resistance of the deposit. Alternative reducing agents include the boranes, dimethylamineborane (DMAB), for example. Use of a borane reducing agent introduces boron into the deposit.
A typical bath for electroless deposition of Damascene barrier layers comprises 0.1 M cobalt chloride or sulfate, 0.2 M sodium hypophosphite, 0.03 M sodium tungstate, 0.5 M sodium citrate, 0.5 M boric acid, and a small amount of a surfactant. Such Co(W, P) baths typically operate at about pH 9 and a temperature of 85°-95° C., and may also contain organic additives.
For electroless deposition of cobalt and nickel on dielectric materials, such as silicon oxide, or on metals that are not sufficiently catalytic for the electroless process, such as copper, a seed layer of a catalytic metal is generally employed. Catalytic palladium is typically deposited on silicon dioxide by immersion of the part in an acidic activator solution containing palladium chloride and fluoride ion. The fluoride ion tends to cause dissolution of surface oxides on the substrate so that a displacement layer of palladium is formed. Alternatively, a seed layer of the electrolessly deposited metal, cobalt or nickel, may be applied by sputtering. For electroless deposition of cobalt and nickel on aluminum, the aluminum substrate is first zincated in an alice solution, which provides a zinc surface layer that dissolves in the electroless bath prior to deposition of cobalt or nickel.
Recently, direct deposition of Co(W, P) capping layers on Damascene copper circuits from a bath employing two reducing agents was reported [T. Itabashi, N. Nakano and H. Akahoshi, Proc. IITC 2002, p. 285-287]. In this case, electroless deposition is initiated by the more active reducing agent (DMAB), which is present at a relatively low concentration. As the DMAB reducing agent becomes depleted at the part surface, electroless deposition is sustained by the less active reducing agent (hypophosphite), which provides better deposit properties.
Electroless copper baths are also widely used by the electronics industry to provide conductive seed layers on poorly conductive substrates. Electroless copper baths typically contain copper sulfate, a complexing agent (e.g., EDTA), a reducing agent (e.g., formaldehyde or glyoxilic acid), a stabilizer (e.g., 2,2-dipyridyl), and hydroxide ion (added as sodium hydroxide or tetramethylammonium hydroxide).
Close control of the concentrations of the constituents of electroless plating baths is necessary to provide acceptable deposit properties. Some constituents can be detected by standard analytical techniques whereas specialized methods are needed to measure the concentrations of other constituents. A method for measuring the concentration of reducing agents in electroless plating baths, based on metal electrodeposition rate measurements, is described in U.S. Pat. No. 6,709,561 to Pavlov et al. (issued Mar. 23, 2004)). A method for measuring the concentration of complexing agents in electroless plating baths, based on titration with a metal complexing ion (e.g., La3+) and endpoint detection via a fluoride ion indicator, is described in a U.S. Pat. No. 6,890,758 to Shalyt et al. (issued May 10, 2005).
One important control parameter for electroless plating baths is the bath stability. A compromise is required in that the bath must provide both an acceptable metal deposition rate and resistance to spontaneous decomposition (in the absence of a catalyst). This compromise is inherent in the selection of the bath complexing agent (which tends to stabilize the metal ions in the bath) and the reducing agent (which tends to chemically reduce the metal ions). Generally, for a given bath formulation, bath stability is increased by addition of the complexing agent and is decreased by addition of the reducing agent.
However, other factors also affect the stability of electroless plating baths so that the bath stability typically cannot be predicted based on measurements of the concentrations of the bath complexing agent and reducing agent. In particular, plating baths are usually proprietary formulations that may include surfactants, organic additives (designed to improve the deposit properties) and/or other bath stabilizers, which may comprise additional inorganic complexing agents and/or organic species. The bath makeup and replenishment chemicals are typically provided as proprietary solutions that contain multiple species whose concentrations are not disclosed, making it difficult to determine the effects of individual species. Furthermore, bath breakdown products (especially of organic species) and bath contaminants (derived from substrate materials and/or drag-in, for example) may also affect the bath stability. It is generally impractical to predict the stability of an electroless plating bath by measuring the concentrations of each species involved, especially since the effects exerted may involve interference or synergy.
Consequently, a means of measuring the stability of electroless plating baths is needed. Such a means would enable the bath to be replaced as-needed rather than according to a schedule, reducing the costs and environmental impact of electroless plating processes. In addition, the close process control provided would enable the stability of the bath to be more closely matched to the requirements of the process to improve the deposit properties. For example, the bath stability might be reduced (by increasing the reducing agent concentration or decreasing the bath complexing agent concentration) to improve substrate coverage. Early detection of bath instabilities that affect the quality of the deposit would also reduce the costs and impact of scrap. A bath instability might result, for example, from a bath contaminant or a variation in a control parameter, such as bath temperature.
A recent publication [R. W. M. Kwok, K. C. M. Chan and M. W. Bayes, “Development of an Electroless Nickel Immersion Gold Process for PCB Final Finishes”, Circuit World 30(3), 37-42 (2004)], which is hereby incorporated by reference, describes a method for measuring the stability of an ENIG electroless nickel plating bath “by titrating the bath solution with a palladium solution until nickel starts depositing on the apparatus wall”. This publication introduces a stability index proportional to the amount of palladium titrant needed to produce a nickel deposit. The titration endpoint used for this prior art approach, which involves nickel deposition on a vessel wall, is difficult to detect precisely and renders the titration analysis method time-consuming and difficult to automate. In particular, nickel deposits tend to passivate and are typically removed by dissolution in strong acid solution.
An objective of the present invention is to provide a relatively precise method for measuring the stability of electroless plating baths that can be effected under computer control. Another objective of the invention is to provide an automated apparatus for practicing the method of the invention.